Looking Inside Hidden
Excitons with THz
Radiation
Tim Gfroerer
Davidson College
Supported by the American
Chemical Society – Petroleum
Research Fund
Outline
• Motivation: Excitons
• Using THz radiation
– Beyond broadening
– Internal transitions
• Proposed Experiment:
– Optically-detected THz
resonance (ODTR)
• Application Example:
GaAs:N
What are excitons?
+
-
Hydrogen-like electron-hole pairs bound
by the Coulomb interaction.
Reduced effective mass
Exciton Hamiltonian:
  2 k 2
2K 2
e 2 
H



2(me*  mh* )  2mr* 4 r 
Reduced effective mass:
Energy levels: En, K
E
Eg
1
1
1


mr* me* mh*
2K 2
mr*e 4


*
*
2(me  mh ) 2(4 ) 2  2 n 2
Free
states
E2,K
E1,K
Bound
states
K
Reduced dimensionality
in quantum wells
Barrier
+
Well
Conduction
band
Barrier
Valence
band
Quantum confinement squeezes the exciton
(even in the plane of the quantum well),
increasing the binding energy.
Optical vs. THz Transitions
E
Free
states
Eg
E2p,K
E1s,K
Bound
states
Transitions:
Optical
Phonon
THz
K
Proposed Experiment:
ODTR
Typical ODTR Traces
M. S. Salib et al., PRL 77, 1135 (1996).
Application Example:
GaAs:N
• Original interest: 1eV gap
for quad junction solar cell
• Caveat: multitude of
localized states
• Potential still exists for
broadband light-emitting
devices
[N]-dependent spectra
Wavelength (nm)
840
GaAs:N
1.5 K
19
2.0
S86, 1.0 10
19
S226, 2.3 10
19
S85, 4.5 10
20
S84, 1.0 10
1.5
820
1.4864
2.5
1.468
Absorption L
16
860
1.426
3
S113,
< 10
17
S89, 3.2 10
17
S88, 9.5 10
2
18
S107, 4.4 10
18
S87, 5.4 10
3.0
AbsorptionL
4
1.5135
1.5152 1.5147
1.5067
GaAs:N
1.5 K 1.5093
880
1.4988
Wavelength (nm)
830
825
820
815
1.0
1
0.5
0
0.0
1.49
1.50
1.51
Energy (eV)
1.52
1.40
1.44
1.48
1.52
Energy (eV)
Yong Zhang et al., Phys. Stat. Sol. B
240, 396 (2003).
Electron Effective Mass
0.6
GaAs1-xNx
QW ER
QW ER
PL LW
ODCR
Electron Effective Mass (m0)
0.5
0.4
0.3
EV = 0
0.2
EV > 0
0.1
0.01
0.02
0.03
0.04
Nitrogen Concentration x
Y. Zhang et al., PRB 61, 7479 (2000).
E.D. Jones et al., PRB 62, 7144 (2000).
P.N. Hai et al., APL 77, 1843 (2000).
Conclusions
• Excitons are critical in modern
opto-electronic devices
– Reduced dimensionality increases
binding energy so excitons exist
at room temperature
• THz is an ideal excitonic probe
– Sees through problematic
inhomogeneous broadening
– Induces internal transitions not
accessible in ordinary PL
• THz studies of GaAs:N may
elucidate this important alloy